1. Field of the Invention
This invention relates generally to the field of detecting magnetic fields, and in particular, to detecting the magnetic field resulting from the rotation of large magnets in a medical treatment device.
2. Description of Related Art
Certain medical treatment devices utilize large rotating magnet(s) weighing, for example, several ounces. One example is the Magboy device, which is designed to be rolled over a part of the body and thus generate a magnetic field which can be therapeutic for certain injuries and pathologies. It is important during treatment to be certain that the magnetic fields produced by the device are of sufficient strength as applied to the patients. In some cases, it is important first to demonstrate that such a magnetic field is produced in the first instance. One would expect that a conventional magnetic field detector utilizing a pickup coil, that is a Gauss meter, could be used for monitoring the operation of a Magboy device or a similar device. However, such detectors utilizing pickup coils also detect background EMF fields. EMF is present everywhere in buildings when electrical power is used. One solution is to reduce the sensitivity to the point that no EMF is detected. Unfortunately, it happens that the magnetic field produced by a Magboy device, for example, is much weaker than the background EMF; by a factor of about 1000. Therefore, high sensitivity is critical.
It is also important that a detector be relatively inexpensive to produce and extremely easy to use. Magnetic fields resulting from other kinds of sources need to be detected and a number of detectors of different designs have been developed over the years. Four such detectors are illustrative.
A detector described in U.S. Pat. No. 4,303,886 comprises: magnetic sensor means including first and second windings; drive means, connected to said first winding, for cyclically increasing the magnetic field in said sensor means to a value greater than that value which causes saturation in said sensor means; detection means, connected to said second winding, for providing an output signal whenever the absolute magnitude of signals received from said second winding is greater than a predetermined value; conversion means, connected to said detection means, for providing a bilevel output signal which changes once for each occurrence of an output signal from said detection means; integration means, connected to said conversion means, for providing an apparatus output signal indicative of the non-symmetry history of bilevel output signals received from said conversion means; and, feedback means, connected between said integration means and said drive means, for returning a feedback signal which substantially cancels the effect of an external field being measured. The detection means comprises positive and negative threshold sensors.
A meter for measuring extremely-low-frequency (ELF) electromagnetic fields, described in U.S. Pat. No. 5,150,051, comprises: a coil that produces a coil voltage in response to varying magnetic fields; a filter-integrator circuit connected to receive the coil voltage as its input and having as its transfer function the product of those of a high-pass filter and an integrator, the high-pass filter having a cut-off frequency below 60 Hz and an attenuation at 10 Hz at least 20 db greater than its attenuation at 60 Hz; and, a display circuit responsive to the output of the filter-integrator circuit for generating a visual indication thereof.
A method for measuring a local magnetic signal in the presence of a magnetic noise field, described in U.S. Pat. No. 5,283,522, comprises the steps of: sensing said local magnetic signal and a noise component from said magnetic noise field; modulating said noise component at a first frequency; producing a first signal from said local magnetic signal and said modulated noise component; bandpass filtering said first signal at said first frequency to produce a second signal; demodulating and amplifying said second signal to produce a third signal; and, subtracting said third signal from said first signal to produce said local magnetic signal.
A radiation detector adapted to detect and provide a warning of a presence of radiation having a frequency of about 5 Hertz to about 400,000 Hz, described in U.S. Pat. No. 5,311,130, comprises: coil means for sending said radiation and for producing an alternating electrical current signal when said radiation is sensed; amplifier means, coupled to said coil means, for amplifying said alternating electrical current signal only when said frequency of the sensed radiation is less than about 2,000 Hz, thereby producing an amplified signal; first rectification means, coupled to said coil means, for rectifying said alternating electrical current signal only when said frequency of the sensed radiation is greater than about 2,000 Hz, thereby producing a first direct current signal; second rectification means, coupled to said amplifier means, for rectifying said amplified signal, thereby producing a second direct current signal; summing means, coupled to said first and second rectification means, for summing said first and second direct current signals, thereby producing a summed signal; comparator means, coupled to said summing means, for comparing said summed signal to a predetermined value and for producing a comparison output signal only when said summed signal exceeds said predetermined value; first multi-vibrator means, coupled to said comparator means, for selectively producing a visual alarm signal only when said comparison output signal is produced by said comparator means; and, second multi-vibrator means, coupled to said first multi-vibrator means, for selectively producing an audible alarm signal a predetermined time after said visual alarm signal is produced, thereby cooperating with said first multi-vibrator means to provide a warning of said presence of said radiation.
None of the foregoing references is directed to a method or apparatus which can easily and inexpensively distinguish between the magnetic field produced by a rotating magnet medical treatment device and background EMF, and in so doing, accurately detect and measure the magnetic field from the medical treatment device.